Process for treatment of a contaminated liquid

ABSTRACT

A process for purification treatment of an impure liquid, such as a contaminated condensate, obtained when producing paper or paper pulp. The liquid is introduced intro a stripper device at an admission point (C) and the stripper device is also supplied with an acidifier preferably at a point (D) that is located above the admission point (C) and malodorous gases are driven off from the stripper device.

TECHNICAL FIELD

The present invention relates to a process for purification treatment ofcondensate from the production of paper or paper pulp. In this process,the condensate, or other contaminated liquid, undergoes stripping inorder to remove impurities and malodorous substances. According to theinvention, an acidification takes place in conjunction with thestripping, and this acidification is preferably effected above the pointof admission of the condensate, which fact facilitates the driving-offof the undesired impurities.

STATE OF THE ART AND PROBLEMS

Owing to the increasing interest in the environment and ourunderstanding of the ecological cycle which exists in nature, there is agreat desire, both among consumers and among producers, to decrease thedischarges of pollutants arising from human activities. In recent years,very strenuous efforts have been made to decrease the discharges fromour pulp and paper mills, and major advances have indeed been made. Thedemand of the market for paper which has been bleached without usingchlorine has led to alternative bleaching chemicals, such as ozone,persulphuric acid, peracetic acid and hydrogen peroxide, coming intouse. A goal for pulp producers is to create "the closed mill", in whichdischarges are minimized and return of liquid flows to the fibre line,and chemical recovery, are maximized. In order to be able to achievethis, it is advantageous for the bleaching process not to containelemental chlorine or chlorine dioxide, since, among other reasons,chlorine has a corrosive effect on the equipment and can form toxiccompounds if it is combusted in the recovery boiler.

In other respects, returning washing liquid in countercurrent to thefibre stream also places great demands on the purity of the liquid. Somewashing stages are more sensitive than others to impurities. For thisreason, there is often the need for some form of purification of thedifferent condensates which can be utilized as washing liquid. In thiscontext, a distinction is made between different types of condensatesince the latter have different contents of impurities.

When the black liquor is extracted from the digester, the pressure isreduced somewhat and the liquor is flashed, resulting in steam beingdriven off. The steam is accompanied, for example, by terpenes, methanoland reduced sulphur compounds, which, to a large extent, accompany thesteam which condenses after cooling has taken place. In this way, acooking department condensate is formed from which turpentine isnormally separated off in a decanter.

The black liquor is normally concentrated by evaporation in severalstages. The units, which are termed effects, are numbered in accordancewith the route of steam supply within the plant. Thus, fresh steam issupplied to the first effect and the liquor steam which has beenobtained in preceding stages is used as a heat source in the subsequentstages. This is possible owing to the gradual decline in pressure. Theheat content which remains in the liquor steam from the final effect iscondensed in one or more surface condensers. By means of allowing theliquor steam to condense out step-wise on different heat surfaces in theplant, it is possible to segregate, on the one hand, heavilycontaminated condensates, to be purified, in a stripper column, and, onthe other hand, very pure condensates which can be used directly in themill without odour treatment or some other form of purification.

Cooking department condensate and heavily contaminated condensate fromthe black liquor evaporation is normally conveyed to a steam stripperfor purification. A steam stripper usually separates off methanol,ethanol, terpenes and malodorous sulphur compounds (hydrogen sulphide,methylmercaptan, dimethyl sulphide and dimethyl disulphide, etc.) veryefficiently. The contaminants which have been separated off are obtainedin a concentrated stream which has to be dealt with. It is usuallyconveyed away and combusted. Condensate which has been treated in asteam stripper often has a very low content of COD. Malodorous compoundswhich remain in the stripped condensate are, apart from traces ofhydrogen sulphide, a number of terpenes such as ocimene, alfapinene,delta-3-carene and decanal. These substances do not smell so badly asthe sulphur compounds which are otherwise common in malodorouscondensates.

The evaporation condensates which are not so heavily contaminated areoften termed "pure condensates". The problem is that they neverthelesscontain some malodorous impurities. Recently, several malodoroussubstances, for example dimethyl trisulphide, 2,3-dimethylphenol, and avariety of trithiolanes and tri-sulphides have been discovered in thesecondensates in addition to those which were previously known. It istherefore desirable to purify these condensates as well in order toavoid problems, inter alia with the working environment, when closingthe kraft mill.

Many studies have been carried out to investigate the problem ofcontaminated condensates from the pulp industry, and also the problem ofcontaminated water in general, and many suggestions for solving theseproblems have indeed been presented. The different methods which existcan be classified into the following groups: phase conversion (e.g.stripping and adsorption with active charcoal), biological treatment,thermal or catalytic oxidation and chemical oxidation. The presentinvention relates to the group termed phase conversion, or, to be moreprecise, stripping.

It is known, for example from Tom Burgess, The Basics of Foul CondensateStripping, 1993 Kraft Recovery Operations Short Course, TAPPI Notes,that the efficiency of stripping is affected by temperature and pH. Itis thus an advantage to have a temperature of above 50° C. for strippingwith air and of above 70° C. for stripping with steam. The document alsostates that the pH should be below 7 when stripping with air and thatthe efficiency of stripping with steam depends on pH in the same way aswhen stripping with air.

A method has also been disclosed, in EP 521 308, for combined treatmentof waste containing ammonia and/or hydrogen sulphide and a sodiumhydroxide solution. The NaOH solution is in this case acidified withsulphuric acid to a pH of between 3 and 7 and is then mixed with thewaste. The mixture is heated and is conveyed to a stripper column whereammonia and hydrogen sulphide are driven off with steam. The method isadapted for problem situations in the petrochemical processing industry.

However, practical trials involving acidification of condensate going toa stripper column have shown that the resulting driving-off is not aseffective in practice as theoretical calculations suggest it should be.

SOLUTION AND ADVANTAGES

The present invention provides a process which improves, in an efficientmanner, the treatment of contaminated and malodorous condensate from thepaper industry or pulp industry, so that this condensate can, if sodesired, be re-used in the pulp production operation.

The process is characterized in that a stripper device is supplied notonly with condensate or another impure liquid, but also with anacidifier, preferably at a point situated above the point of admissionof the liquid. The theory behind the invention is that acid (acidifier)is not driven off and cannot therefore migrate upwards in the stripperdevice. In the case where the acid is added at the same point as theimpure liquid, this means that no acidification effect is obtained inthe upper part of the stripper, that is to say in the fortifyer partwhere most of the driving-off occurs. In contrast, the pH falls thefurther down in the stripper one goes. By adding the acidifier in theupper part, the fortifyer part or reflux condenser, this problem isavoided and it is possible to achieve an efficient driving-off of, forexample, sulphur compounds and methanol. The acidifier is preferablymixed directly into the liquid reflux from a reflux condenser which iscoupled to the top of the stripper device. It is more preferable stillfor the acidifier to come into contact with outgoing stripper steam, forexample in a countercurrent contact device where the acidifier sprinklesdown and meets the ascending stripper steam. The acidifier is thereaftermixed, either completely or in part, into the liquid reflux.

It is known, from EP 0 038 686 A2, to add for example SO₂ or SO₃ to adistillation column at a point above the admission point, whereby carbondioxide is driven off from the binary mixture of carbon dioxide andhydrogen sulphide. The process differs from the present invention inthat it is not malodorous gases which are driven off and also in thatthe problem to be solved is another one. In the EP document, the degreeof malodorous gases (hydrogen sulphide) which are driven off aredecreased and the hydrogen sulphide is instead concentrated in theexiting condensate. Thus, the possibility of reusing the condensate asprocess water in the plant or of discharging it, is excluded. Thispossibility is a main object with our invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a stripper device system of the presentinvention.

The method will be described below with reference to FIG. 1, which showsa preferred embodiment of the invention.

Feature number 1 in FIG. 1 indicates a stripper device for treatment ofcondensate or other contaminated liquid. The fortifyer part is called Aand the driving-off part is called B. The liquid (2) is introducedbetween the driving-off part and the fortifyer part at an admissionpoint C. A drop in pressure takes place in conjunction with the liquidbeing introduced into the stripper. It is advantageous in this case ifthe incoming liquid has been heated, for example by means of heatexchange (7) with the outgoing, purified liquid (6). Volatile substancesthen leave more easily. Steam (3) is added to the stripper device at itsbottom part, and this steam acts on the impure liquid so that volatilesubstances are driven off with the outgoing stripper steam (4). In orderto achieve an efficient driving-off, it is customary to install a refluxcondenser (8) and return some parts (5) of the stripper steam, which hascondensed out, to the fortifyer part A. The reflux condenser can be madeup of one or more heat exchangers.

According to the invention, an acidification will be carried out byaddition of, for example, carbon dioxide, carbon monoxide, phosphoricacid, sulphur dioxide, sulphuric acid, sulphite, bisulphite or effluentfrom an acid bleaching stage.

The addition point D for the acidifier (9) should in this case be abovethe admission point C, so that acidification is actually obtained at thepoint where it is most useful. The acidifier is preferably added to theliquid reflux (5) from the reflux condenser. It is still more preferablefor the acidifier to be brought into contact with outgoing strippersteam (4) by addition at the point D', for example in a smallcountercurrent contact tower (10) (which is not therefore obligatory)where the acidifier sprinkles down and meets the ascending strippersteam. The acidifier is then mixed into the liquid reflux (5), eithercompletely or in part. The acidification is expediently continued to apH of below 7 in the upper part of the stripper, or even more preferablyto a pH of below 6.

The purified liquid (6) can be used, for example, in washing stageswhich place relatively high demands on purity and freedom from odour,for example lime sludge dilution, lime sludge washing orperoxide-containing bleaching stages.

An advantage of the present process is that since the flow of liquid inthe reflux condenser and the fortifyer part is normally considerablyless than the flow of incoming condensate, a smaller amount of acidifieris used up if the addition takes place in either one of these parts,compared with the situation if the addition were to take place directlyin the condensate or in the lower part of the stripper.

A further advantage of the condensate treatment, over and above the factthat it provides the opportunity of re-using condensate in theoperation, thus simplifying matters as the mill becomes increasinglyclosed, is that the working environment is improved, as well as theenvironment around the mill. As matters stand at present, people livingin the vicinity of the pulp mill can sometimes be greatly inconveniencedby malodorous gases emanating from said pulp mill.

An additional advantage is that it is possible, for example whenmanufacturing packaging material for food and drinks, to utilize plantcondensate in the washing stages on the fibre line without the productsacquiring a residual taste or odour.

With the present invention it is possible to purify condensates which,viewed in relative terms, already belong to the purer condensates in ahypothetical pulp mill before they are re-used in the operation, or morecontaminated condensates before they are conveyed, for example, tobiological purification and discharge, or to the dissolver tank forcausticizing.

The invention can of course also be applied to any other process waterand also, for example, to effluent from a bleaching stage before it isconveyed further on in the operation.

The process can be applied directly to existing stripper devices, withonly minor alterations. Another interesting application is in completelynew purification stages for treatment of semi-soiled condensate. It ispossible in this case to carry out stripping with a low quotient ofsteam per kilogram of added condensate, preferably under 0.10 kg/kg, forexample 0.03 to 0.04 kg/kg instead of the usual 0.16 to 0.20 kg/kg whenstripping cooking department condensate and soiled evaporationcondensate. If acidification according to the invention is implemented,larger amounts of hydrogen sulphide present in the condensate can bedriven off in controlled forms and conveyed away for destruction. Therisk of hydrogen sulphide escaping in the bleaching department, inconjunction with acidification, is therefore much smaller. Otherwise,this can be a problem which arises as the plant becomes increasinglyclosed.

Although it is preferred to add the acidifier to the upper part of thestripper device, it is possible to contemplate acidifying the condensatedirectly before the latter is conveyed into the stripper. Ifappropriate, acidification can already be carried out in associationwith the condensing-out of the steam which forms the condensate, forexample from digester flashing or evaporation.

An alternative addition can be one or more electrolytes. It is alreadyknown that by adding electrolyte/electrolytes, the phase equilibrium canbe modified in a manner favourable to separation. When addingelectrolyte, a distinction is made as to whether the relative volatilityof a component increases or decreases. For mixtures of water and alcohol(for example methanol), the relative volatility of the alcohol increasesin most cases. When purifying condensate, it is possible to use, forexample, Na⁺ ions, with associated counter-ions as electrolyte.

The invention is not limited to the embodiments which are describedhere, and can instead be varied within the scope of the patent claimswhich follow. Thus, the person skilled in the art will readilyappreciate that the technology can also be applied to a stripper withoutfortifyer part, or to a device for stripping with air. The impure liquidcan of course be admitted to the stripper device at more than one point.In such a case, it is preferable for the acidifier to be added above theuppermost point, although such an addition can also have a favourableeffect if carried out at an intermediate point.

We claim:
 1. A process for purification treatment of a contaminatedliquid obtained when producing paper or pulp, the processcomprising:producing the contaminated liquid; providing an acidifierfluid, a stripper device having an admission point and an acidifierpoint that is located above the admission point and a liquid reflux anda reflux condenser that is in fluid communication with the stripperdevice; introducing the contaminated liquid into the stripper device atthe admission point thereof; supplying the acidifier fluid to the liquidreflux; supplying the stripper device with the acidifier fluid at theacidifier point; and driving off malodorous gases from the stripperdevice.
 2. The process according to claim 1 wherein the process furthercomprises the steps of bringing the acidifier fluid into contact with asteam provided by the stripper device before the steam is permitted tocondense out in the reflux condenser.
 3. A process for purificationtreatment of a contaminated liquid obtained when producing paper orpulp, the process comprising:producing the contaminated liquid;providing an acidifier fluid by selecting a sulfuric acid or an effluentobtained from an acid bleaching stage, a stripper device having anadmission point and an acidifier point that is located above theadmission point; introducing the contaminated liquid into the stripperdevice at the admission point thereof; supplying the stripper devicewith the acidifier fluid at the acidifier point; and driving offmalodorous gases from the stripper device.
 4. The process according toclaim 3 wherein the process further comprises the steps of introducingelectrolytes to increase a relative volatility of contaminants includedin the malodorous gases that are to be driven off.